We know that you probably don't need help figuring out if what you're drinking is hot or cold, but that only makes these Smart Coasters all the more endearing. Essentially a solar cell trapped in waterproof resin, cold drinks light up blue and hot drinks light up red. Full instructions to build your own can be found in The Hungry Scientist Handbook (which runs about $12 at Amazon). But for the less labor-inclined, sticking your finger into visitors' drinks has always been considered a fine gesture of hospitality. [Evil Mad Scientist Lab via boingboing]